1. Field of the Invention
The invention involves systems and methods for calibrating measurements made by catheters usable in medical evaluations of a condition of a living body.
2. Related art
Evaluating cardiac function and hemodynamics is an integral component in managing patients with heart disease. Various invasive and noninvasive techniques have been used to assess volumes inside the hearts of these patients and include echocardiography, contrast angiography, radionuclide ventriculography, computed tomography, and magnetic resonance imaging.
However, none of these methods enables instantaneous and beat-by-beat assessment of cardiac cavitary volume.
A method for examining instantaneous changes in cavitary volume was previously developed on the basis of a conductance catheter carrying about 10 equidistant ring electrodes along the catheter shaft. The method involves delivering a very small alternating current between a pair of ring electrodes located at the distal and proximal ends of the catheter along the major axis of the heart cavity (such as the left ventricle), and recording resulting potentials between pairs of ring electrodes located on the catheter shaft between the current-delivering ring electrodes. Conductance (defined as 1/impedance) is the ratio of delivered current to measured varying potential. During the heartbeat, continuous changes in cavitary cross-sectional areas perpendicular to the catheter shaft (i.e. segmental volumes) cause instantaneous changes in conductance between catheter electrodes. Since injected current is fixed externally, measured potentials are inversely related to volume changes between recording electrode pairs. Hence, the conductance catheter method has been successfully used in recording instantaneous volume signals and assessing ventricular function. However, the conductance catheter method requires additional laborious steps for calibrating conductance signals into volume, which include (1) applying the thermodilution method to determine peak-to-peak change in cavitary volume (i.e. stroke volume), and (2) using the hypertonic saline infusion method to estimate a fixed value for parallel conductance brought about by electric current leakage into the myocardium and adjacent cavities. Calibration of conductance signals is adversely affected by procedural errors in these steps and by theoretical error in assuming constant parallel conductance effect throughout the heartbeat. Meanwhile, locations of current injecting and potential recording electrodes inside the heart cavity impacts the quality and accuracy of conductance signals. Yet, conductance signals have been predominantly recorded without accurate knowledge of electrode locations inside the heart cavity.